High power broad area and single spatial mode semiconductor lasers are used in a variety of applications that require some stability of the wavelength over a range of a few nanometers. Examples include broad area laser pumping of solid-state crystals such as Nd:YAG which requires a pump wavelength of about 808±3 nm, broad area laser pumping of ytterbium doped fiber amplifiers and lasers which require pump wavelengths of about 970±1 nm, or single spatial mode laser pumping of erbium doped fiber amplifiers (EDFA) which requires a pump wavelength of about 975±0.5 nm.
Note that with this type of pump source there is no need for single frequency operation. Multiple longitudinal laser modes within a few nanometer wide envelope is acceptable or in some cases even desirable.
Another problem is that changes in temperature, laser drive current and power or optical feedback can shift the laser diode wavelength out of the specified range.
One common method to control the wavelength is to use a periodic grating, such as in distributed Bragg reflector (DBR) or distributed feedback Bragg (DFB) lasers. However most work on gratings has been to achieve single frequency operation for relatively low power lasers.
For instance, Kagawa et al. (U.S. Pat. No. 4,984,242) disclose an aluminum gallium arsenide (AlGaAs) semiconductor gain-coupled DFB laser which operates in single longitudinal mode. The lateral grating, which modulates the width of the current injected region, extends over the whole length of the laser, as illustrated in FIG. 1b. 
On the other hand, Okuda et al. (U.S. Pat. No. 5,568,505) disclose a similar device in the indium gallium arsenide phosphide (InGaAsP) material system emitting at a wavelength of 1310 nm. In order to optimize their laser structure for analog modulation performance, which requires very linear single mode performance characteristics (low 3rd harmonic intermodulation distortion), they implement a grating with a coupling coefficient by cavity length product between 0.4 and 1.0 in value. The grating in some of the embodiments they present can be partial, i.e. occupying only a portion of the cavity length and may incorporate a quarter-wave shifted grating. Their device is optimised to relatively low optical output powers, below about 10 mW.
Another example is a wavelength stabilized single mode laser in the aluminum indium gallium arsenide phosphide (AlInGaAsP) material system as disclosed by Fukunaga (U.S. Pat. No. 5,602,866). They describe a photonic integrated circuit with three sections on a single waveguide incorporating a DBR laser, a modulator and an amplifier. Their approach is to generate a single mode lasing frequency at relatively low power, modulate it with a signal and then optically amplify it for high power output. However, they are silent on some key parameters, which are essential for high power pumping operation, such as, suitable values for front and rear facet reflectivity, cavity length, and product of coupling coefficient-cavity length.
Botez et al. (U.S. Pat. Nos. 6,195,381 and 6,363,092) teach a broad-area DFB laser for optical pumping applications. However, since the grating extends for the total length of the lasing cavity, practical limitations exist to keeping the coupling coefficient to adequately low values. As their used cavity length is longer than 1.0 mm, Botez et al. recommend a coupling coefficient—length product of about unity. As is known in the art, DFB can lase in one of two modes, depending on the coupling coefficient value. Avoiding this ambiguity can restrict the coupling coefficient values.
Muroya (U.S. Pat. No. 6,577,660) describes a laser diode based on the InGaAsP material system. This system is known to possess intrinsic electronic mechanisms, which differ fundamentally from those of the AlGaAs system, so they must be optimised in a totally different way to achieve high output optical power. For this reason, Muroya describes a relatively short DFB structure, about 300 micron in length and incorporating a quarter-wave shifted grating which is advantageous for, among other things, reducing the internal peak power density. The technology for fabricating such a grating is rather more complex. The emission wavelengths achievable in this system are also not suitable for the pumping applications which are a major aspect of this disclosure.
Nakamura (U.S. Pat. No. 6,678,302) employs the same material system as Muroya, but focuses on integrating the laser diode with a modulator. However, he does not specify suitable values for the product of coupling coefficient and cavity length.
This invention addresses the above problems by implementing a low-loss intra-cavity grating while achieving moderate wavelength stability of a broad-area or single spatial mode laser diode without adversely affecting the laser diode efficiency or maximum optical power.